The problem#
I use KeePassXC as my password manager in Firefox and while sometimes the connection between Firefox and KeePassXC drops and I have to explicitly click reconnect, it recently stopped working entirely.
The solution#
Install the keepassxc-full package instead of
the keepassxc package. If you get the browser extension
via the webext-keepassxc-browser
package, then your package manager will automatically get the right one.
(This only applies to Debian Sid and Trixie or newer.)